Investigating the Nature of the Active Species in ... - ACS Publications

Organometallics 2009, 28, 6003–6013 DOI: 10.1021/om900490b

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Investigating the Nature of the Active Species in Bis(imino)pyridine Cobalt Ethylene Polymerization Catalysts Igor E. Soshnikov,†,‡ Nina V. Semikolenova,† Alexey N. Bushmelev,†,‡ Konstantin P. Bryliakov,†,‡ Oleg Y. Lyakin,†,‡ Carl Redshaw,§ Vladimir A. Zakharov,† and Evgenii P. Talsi*,†,‡ †

Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090, Novosibirsk, Russian Federation, ‡Novosibirsk State University, 630090, Novosibirsk, Russian Federation, and §School of Chemistry, University of East Anglia, Norwich NR4 7TJ, U.K. Received June 10, 2009

The activation of bis(imino)pyridine cobalt-based catalysts of ethylene polymerization with methylalumoxane (MAO), AlMe3, and AlMe3/[CPh3][B(C6F5)4] has been studied by 1H, 2H, and 19 F NMR and EPR spectroscopy. The nature of the active sites of polymerization is discussed. The polymerization kinetics and the polymer properties for the different catalyst/activator systems were correlated with spectroscopic data. Introduction In the past decade, there has been much interest in iron and cobalt ethylene polymerization catalysts LMIICl2/MAO and LMIICl2/AlR3, where M=Fe or Co, L=bis(imino)pyridine

ligand, and R=alkyl.1-38 NMR spectroscopic investigations define the structures of iron species formed in the LFeIICl2/ MAO and LFeIICl2/AlMe3 systems.10,17,38 It was shown that ion pairs [LFeII(μ-Me)2AlMe2]þ[Me-MAO]- predominate

*Corresponding author. Fax: þ7 383 3308056. E-mail: talsi@ catalysis.ru. (1) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143– 7144. (2) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049–4050. (3) Bennett, A. M. A. (Dupont) WO 98/27124, 1998; Chem. Abstr. 1998, 129, 122973x. (4) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849–850. (5) Bennett, A. M. A. CHEMTECH 1999 (July), 24-28. (6) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Stromberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728–8740. (7) Britovsek, G. J. P.; Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.; Redshaw, C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood, M. R. J. Chem.;Eur. J. 2000, 6, 2221–2231. (8) Ma, Z.; Qui, J.; Xu, D.; Hu, Y. Macromol. Rapid Commun. 2001, 22, 1280–1283. (9) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Mastroianni, S.; Redshaw, C.; Solan, G. A.; White, A. J. P. J. Chem. Soc., Dalton Trans. 2001, 1639–1644. (10) Talsi, E. P.; Babushkin, D. E.; Semikolenova, N. V.; Zakharov, V. A. Macromol. Chem. Phys. 2001, 202, 2046–2051. (11) Kooistra, T. M.; Knujenburg, Q.; Smits, J. M. M.; D. Horton, A.; Budzelaar, P. H. M.; Gal, A. W. Angew. Chem., Int. Ed. 2001, 40, 4719–4722. (12) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.; White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252–2253. (13) Abu-Surrah, A. S.; Lappalainen, K.; Piironen, P.; Lehmus, P.; Repo, T.; Leskela, M. J. Organomet. Chem. 2002, 648, 55–65. (14) Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P.; Babushkin, D. E.; Sobolev, A. P.; Echevskaja, L. G.; Khysniyarov, M. M. J. Mol. Catal. A 2002, 182-183, 283–294. (15) Chen, Y.; Qian, C.; Sun, J. Organometallics 2003, 22, 1231–1236. (16) Paulino, I. S.; Schuchardt, U. J. Mol. Catal. A 2004, 211, 55–58. (17) Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. Organometallics 2004, 23, 5375–5378. (18) Zhang, Z.; Chen, S.; Zhang, X.; Li, H.; Ke, Y.; Lu, Y.; Hu, Y. J. Mol. Catal. A 2005, 230, 1–8.

(19) Liu, J.-Y.; Zheng, Y.; Li, Y.-G.; Pan, L.; Li, Y.-S.; Hu, N.-H. J. Organomet. Chem. 2005, 690, 1233–1239. (20) Tellmann, K. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J. Organometallics 2005, 24, 280–286. (21) Humphries, M. J.; Tellmann; Gibson, V. C.; White, A. J. P.; Williams, D. J. Organometallics 2005, 24, 2039–2050. (22) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406–3408. (23) Barabanov, A. A.; Zakharov, V. A.; Semikolenova, N. V.; Echevskaja, L. G.; Matsko, M. A. Macromol. Chem. Phys. 2005, 206, 2292–2298. (24) Scott, J.; Gambarotta, S.; Korobkov, I.; Knijnenburg, Q.; de Bruin, B.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 17204–17206. (25) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones, B. B.; Fish, B. M.; Schiffhauer, M. F. Organometallics 2006, 25, 2978–2992. (26) Gibson, V. C.; Long, N. J.; Oxford, P. J.; White, A. J. P.; Williams, D. J. Organometallics 2006, 25, 1932–1939. (27) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901–13912. (28) Barabanov, A. A.; Bukatov, G. D.; Zakharov, V. A.; Semikolenova, N. V.; Mikenas, T. B.; Echevskaja, L. G.; Matsko, M. A. Macromol. Chem. Phys. 2006, 207, 1368–1375. (29) Kissin, Y. V.; Qian, C.; Xie, G.; Chen, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6159–6170. (30) Redshaw, C.; Gibson, V. C.; Solan, G. A. Chem. Rev. 2007, 107, 1745–1776. (31) Bart, S. C.; Lobkovsky, E.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2007, 46, 7055–7063. (32) Sun, W. H.; Zhang, S.; Zuo, W. C. R. Chim. 2008, 11, 307–316. (33) Hanton, M. J.; Tenza, K. Organometallics 2008, 27, 5712–5716. (34) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones, B. B.; Fish, B. M.; Schiffhauer, M. F. Organometallics 2008, 27, 1147–1156. (35) Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J. Organometallics 2008, 27, 1470–1478. (36) Fernandez, I.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Organometallics 2008, 27, 109–118. (37) Barabanov, A. A.; Bukatov, G. D.; Zakharov, V. A.; Semikolenova, N. V.; Echevskaja, L. G.; Matsko, M. A. Macromol. Chem. Phys. 2008, 209, 2510–2515. (38) Bryliakov, K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov, V. A. Organometallics 2009, 28, 3225–3232.

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Organometallics, Vol. 28, No. 20, 2009

Soshnikov et al.

Scheme 1. Structures of Cobalt Precatalysts Studied

in the LFeIICl2/MAO systems at [Al]/[Fe] > 500, whereas activation of LFeIICl2 with AlMe3 leads to neutral species of the type L(•-)Fe(þ)(μ-Me)2AlMe2 at [Al]/[Fe]>30, where the FeII is coordinated to the anion radical of L.38 As for the LCoIICl2/MAO and LCoIICl2/AlMe3 analogues, the structures of the intermediates formed in these systems are more controversial. According to results of Gibson and Gal, in the LCoIICl2/MAO system, initial cobalt(II) precatalyst reduction to cobalt(I) halide is followed by conversion to a cobalt(I) methyl and ultimately to a cobalt(I) cationic species. Addition of ethylene affords an ethylene adduct [LCoI(ηC2H4)]þ[Me-MAO]-, which is considered to be the immediate precursor to the active species.11,21,30 In contrast, our 1H and 2 H NMR studies of the L2MeCoIICl2/MAO system (Scheme 1) showed that a cobalt(II) complex with proposed structure L2MeCoIIMe(Cl)(MAO) (1) strongly predominates in the reaction solution at 20 °C at least several hours after mixing L2MeCoIICl2 and MAO.14 In this paper, we describe 1H, 2H, and 19F NMR characterization of cobalt(II) and cobalt(I) species formed in the systems LCoIICl2/MAO, LCoIICl2/AlMe3/[CPh3][B(C6F5)4], and LCoIICl2/AlMe3, where L = L2iPr, L3Me, LtBu, and LCF3 (Scheme 1). Correlations between the structure of the observed cobalt species and the structure of the resulting polyethylene are discussed.

Results and Discussion Activation of L2iPrCoIICl2 with MAO and AlMe3/[CPh3][B(C6F5)4]. The 1H NMR spectra of the catalyst systems L2iPrCoIICl2/MAO ([Al]:[Co] = 100:1, [Co] = 5 10-3 M, toluene-d8) and L2iPrCoIICl2/AlMe3/[CPh3][B(C6F5)4] ([Al]: [Co]:[B] = 10:1:2, [Co] = 10-2 M, toluene-d8), recorded at various times after mixing the reagents at -30 to þ20 °C, show that the starting cobalt complex L2iPrCoIICl2 quantitatively converts to cobalt(II) complexes [L2iPrCoIIMe(S)]þ[Me-MAO]- (2) and [L2iPrCoIIMe(S)]þ[B(C6F5)4]- (20 ), respectively (S=solvent or vacancy). The 1H NMR spectra of 2 and 20 are very similar (Figures 1a,b and Table 1). The broad peak at ca. δ 150 in the 1H NMR spectra of 2 and 20 belongs to a Co-CH3 group. A similar resonance was previously observed for the related complex 1 formed in the L2MeCoIICl2/MAO system. Its assignment to the Co-CH3

group was confirmed by 2H NMR spectroscopy, using deuterated MAO.14 As was mentioned, complex 1 was previously assigned to a neutral complex of the type L2MeCoIIMe(Cl)(MAO).14 However, the following data provide evidence in favor of ionic structures for complexes 2, 20 , and 1. AlMe3/[CPh3][B(C6F5)4] is a well known ion pair generating reagent. The 19F spectrum of 20 is typical for an outersphere perfluoroaryl borate anion [B(C6F5)4]- (19F NMR, -20 °C: δ -134.8 (Δν1/2 =40 Hz), -155.0 (Δν1/2 =430 Hz), -165.1 (Δν1/2 = 120 Hz)).39,40 Hence, 20 is the ion pair [L2iPrCoIIMe(S)]þ[B(C6F5)4]- (S=solvent or vacancy). This assumption is in agreement with recent publications of Macchioni and co-workers, demonstrating that for the post-metallocene precatalysts even with the B(C6F5)3 activator outer-sphere ion pairs can be formed and that the vacant coordination site can be substantially unoccupied or involved in weak interaction with solvent.41,42 The [B(C6F5)4]- counteranion of 20 is far from the perturbing paramagnetic center, and therefore its 19F chemical shifts are close to those for diamagnetic ion pairs. Apparently, the related complex 2 is the ion pair [L2iPrCoIIMe(S)]þ[Me-MAO]-. It is evident that the counteranion [Me-MAO]- is placed in the outer coordination sphere of cobalt, since in the case of the direct contact of the oligomeric molecule of [Me-MAO]- with cobalt (an inner-sphere ion pair), extremely broad 1H resonances are expected.43-45 The data presented show that complex 1 (39) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623. (40) Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840–842. (41) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. J. Am. Chem. Soc. 2008, 130, 10354–10368. (42) Ciancaleoni, G.; Fraldi, N.; Budzelaar, P. H. M.; Busico, V.; Macchioni,A. Dalton Trans. 2009, available online. (43) Babushkin, D. E.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 2000, 201, 558–567. (44) Bryliakov, K. P.; Bochmann, M.; Talsi, E. P. Organometallics 2004, 23, 149–152. (45) Bryliakov, K. P.; Semikolenova, N. V.; Yudaev, D. V.; Zakharov, V. A.; Brintzinger, H. H.; Ystenes, M.; Rytter, E.; Talsi, E. P. J. Organomet. Chem. 2003, 683, 92–102.

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previously assigned to L2MeCoIIMe(Cl)(MAO) is in reality the ion pair [L2MeCoIIMe(S)]þ[Me-MAO]-. It is worth noting that the structure of the ion pair [L2iPrCoIIMe(S)]þ[Me-MAO]- (2), formed in the L2iPrCoIICl2/MAO system, differs from the structure of the ion pair [L2iPrFeII(μ-Me)2AlMe2]þ[Me-MAO]- formed in the L2iPrFeIICl2/MAO analogue.38 In the first case, the vacant coordination site of the cobalt is unoccupied or bound by solvent (toluene), whereas in the second case this site is occupied by AlMe3. Complexes 2 and 20 are the major cobalt species in the reaction solution even one day after mixing the reagents at 20 °C. However, prolonged storing (several days) of the samples L2iPrCoIICl2/MAO ([Al]:[Co] = 100:1, [Co] = 5 10-3 M, toluene-d8) and L2iPrCoIICl2/AlMe3/ [CPh3][B(C6F5)4] ([Al]:[Co]:[B] = 10:1:2, [Co] = 10-2 M, toluene-d8) at room temperature results in the gradual decrease of the concentration of complexes 2 and 20 and the growth of the concentration of diamagnetic cobalt(I) complexes 3 and 30 , respectively. 1 H NMR spectra of 3 and 30 (Figure 2, Table 1) display sharp resonances of the L2iPr ligand in the range typical for diamagnetic species.

Figure 1. 1H NMR spectra (20 °C) of the samples L2iPrCoIICl2/ MAO ([Al]:[Co] = 100:1, [Co] = 5 10-3 M, toluene-d8) (a) and L2iPrCoIICl2/AlMe3/[CPh3][B(C6F5)4] ([Al]:[Co]:[B]=10:1:2, [Co] = 10-2 M, toluene-d8) (b), recorded 16 h after mixing the reagents. The inset of the precatalyst is shown for convenience of the line assignment.

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1

H NMR spectra of this type were previously reported for the ion pairs [L2iPrCoI(η-C2H4)]þ[MeB(C6F5)3]- (3C2H4) and [L2iPrCoI(η-N2)]þ[MeB(C6F5)3]- (3N2) (Table 1).21 The 19 F spectrum of 30 is typical for the outer-sphere perfluoroaryl borate anion [B(C6F5)4]- (19F NMR, 20 °C: δ -130.2 (Δν1/2=30 Hz), -161.0 (Δν1/2=280 Hz), -164.1 (Δν1/2=120 Hz)).39,40 On the basis of their 1H and 19F NMR spectra, 3 and 30 can be assigned to the ion pairs [L2iPrCoI(S)]þ[MeMAO]- and [L2iPrCoI(S)]þ[B(C6F5)4]-, respectively. The activation of L2iPrCoIICl2 with MAO and AlMe3/ [CPh3][B(C6F5)4] at 20 °C thus results in the quantitative formation of the cobalt(II) ion pairs (2 and 20 ), which slowly reduce to the ion pairs of cobalt(I) (3 and 30 ) (Scheme 2). Two weeks after the reaction onset at 20 °C, the sample L2iPrCoIICl2/MAO ([Al]:[Co] = 100:1, [Co] = 5 10-3 M, toluene-d8) contains complexes 2 and 3 ([2]:[3] ≈ 1:1). Addition of ethylene to this sample ([Co]:[C2H4] = 1:25, 20 °C)

Figure 2. 1H NMR spectra (20 °C) of the samples L2iPrCoIICl2/ MAO ([Al]:[Co]=100:1, [Co]=510-3 M, toluene-d8) 7 days after storing at room temperature (a) and L2iPrCoIICl2/ AlMe3/[CPh3][B(C6F5)4] ([Al]:[Co]:[B]=10:1:2, [Co]=10-2 M, toluene-d8) 7 days after storing at room temperature (b).

Table 1. 1H NMR Data for the Complexes Observed upon Activation of L2iPrCoIICl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3 (toluene-d8, 20 °C)

complex 2iPr

II

a

L Co Cl2 [L2iPrCoIIMe(S)]þ[Me-MAO][L2iPrCoIIMe(S)]þ[B(C6F5)4][L2iPrCoI(S)]þ[Me-MAO][L2iPrCoI(S)]þ[B(C6F5)4]L2iPrCoI(μ-Me)(μ-Cl)AlMe2b L2iPrCoI(μ-Me)2AlMe2b [L2iPrCoI(η-C2H4)]þ[AlMe3Cl]-b [L2iPrCoI(η-N2)]þ[MeB(C6F5)3]-c [L2iPrCoI(η-C2H4)]þ[MeB(C6F5)3]-c L2iPrCoIMe c L2iPrCoICl c

2 20 3 30 4a 4b 5 3N2 3C2H4

A

B

C

D

E

F

G

H

Py-Hm

Py-Hp

Ar-Hm

Me2CH

Ar-Hp

NdC(Me)

Me2CH

Me2CH

116.5 19.4 19.1 7.71 7.48 NA 7.5 NA 6.69 ∼7.0 7.86 6.91

49.4 45.9 44.3 8.35 7.92 8.1 9.7 8.1 7.55 8.00 10.19 9.54

10.0 NA NA NA NA NA NA NA 6.87 6.77 7.37 7.27

-17.8 -2.4 -2.2 1.20 1.15 NA 1.1 1.1 1.05 1.09 1.19 1.18

-8.8 12.4 12.3 NA NA NA NA NA 7.00 ∼7.0 7.49 7.41

4.4 -23.6 -23.3 -0.80 -0.81 -0.07 N/O 0.6 1.11 0.75 -1.14 0.05

-19.0 -15.2 -14.6 1.03 0.91 0.8 0.6 0.8 0.98 0.76 0.62 1.06

-84.6 -41.5 -40.0 2.44 2.44 3.1 3.1 3.1 2.85 3.02 3.13 3.33

Co-Me

μ-Me

∼152 ∼143 -1.6 -2.8 N/O N/O N/O

a Solution in CH2Cl2. b Spectra recorded at -10 °C, only broad singlets in the each case. c The data are from ref 21. N/O, not observed. NA, not reliably assigned.

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leads to ethylene polymerization, monitored by the decrease of the ethylene peak at δ 5.2. In addition, the resonances of 2 disappear and only 3 is observed in the sample containing ethylene. Hence, ethylene induces the rapid reduction of 2 into 3. Thus, in agreement with the previous assumption,11,21 the cobalt(I) species seems to be the active polymerizing species in the catalyst systems LCoIICl2/MAO. However, the quantitative conversion of cobalt(II) to cobalt(I) occurs only in the presence of monomer. Activation of L2iPrCoIICl2 with AlMe3. In contrast to the L2iPrCoIICl2/MAO and L2iPrCoIICl2/AlMe3/[CPh3][B(C6F5)4] systems, where cobalt(II) species predominate in the reaction solution at least several hours after the reaction onset, the activation of L2iPrCoIICl2 with AlMe3 ([Al]:[Co]=4:1, [Co]= 10-2 M, toluene-d8) leads to a very rapid and quantitative reduction of the starting complex L2iPrCoIICl2 to two types of diamagnetic complexes of cobalt(I), denoted as 4a and 4b (Figure 3a, Table 1). On the basis of the previous studies of Gibson and Gal,11,21 one can expect that complexes L2iPrCoIMe and/or L2iPrCoICl are formed in the L2iPrCoIICl2/ AlMe3 system. However, the 1H NNR spectra of 4a and 4b differ from those of L2iPrCoIMe and L2iPrCoICl (Table 1). We have proposed that 4a and 4b are adducts of the type L2iPrCoI(μ-Me)(μ-Cl)AlMe2 (4a) and L2iPrCoI(μ-Me)2AlMe2 (4b). In agreement with this prediction, only 4b was observed in the sample L2iPrCoCl2/AlMe3 with a [Al]:[Co] ratio of 50. The 1H NMR spectrum of Figure 3a displays two low-field resonances at δ -1.6 and -2.8. The sample containing only 4b exhibits only the one signal at δ -2.8. The latter signal can be attributed to the μ-Me group of 4b, and the signal at δ -1.6 to a μ-Me group of 4a. To support this assignment, the 2 H NMR spectrum of the system L2iPrCoIICl2/Al(CD3)3 ([Al]:[Co] = 6:1, [Co] = 2 10-2 M, toluene) was recorded. This spectrum exhibits resonances at δ -1.7 and -3.0, identical within accuracy of our measurements to those observed in the 1H NMR spectrum. Thus, 4a and 4b are

Soshnikov et al.

neutral cobalt(I) species of the type L2iPrCoI(μ-Me)( μCl)AlMe2 and L2iPrCoI(μ-Me)2AlMe2 (Scheme 2). Complexes 4a and 4b are unstable and rapidly (within several minutes) decay at 20 °C via transfer of the L2iPr ligand to aluminum to afford the paramagnetic complex [L2iPr(•-)Al(þ)Me2], exhibiting an EPR signal at g = 2.003 (Figure 4). This complex was first discovered by Gambarotta and co-workers.24 It is formulated as a complex of AlIII with the anion radical of L2iPr(•-) and two Me anions. The EPR spectrum of Figure 4 displays partially resolved multiline hyperfine splitting (hfs) from Al, N, and H nuclei (inset). This

Figure 3. 1H NMR spectra of the samples L2iPrCoIICl2/AlMe3 ([Al]:[Co] = 4:1) (a) and L2iPrCoIICl2/AlMe3/C2H4 ([Al]:[Co]: [C2H4]=10:1:50) (b) recorded 10 min after mixing the reagents ([Co]=10-2 M, toluene-d8, -20 °C). Sample in “b” 10 min after additional storing at -10 °C (c).

Scheme 2. Interaction of L2iPrCoIICl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3

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Figure 4. EPR spectrum (-196 °C) of the sample L2iPrCoIICl2/ AlMe3 ([Al]:[Co] = 4:1, toluene-d8, [Co] = 5 10-3 M) after 1 day storage at room temperature (asterisk marks signal of an impurity in the sample tube).

splitting has been previously well resolved and interpreted for the same aluminum complex formed upon reacting the ligand, L2iPr, with AlMe3.24 The EPR spectrum of Figure 4 also displays a very broad and intense resonance (ΔH1/2 =1700 G) from the ferromagnetic cobalt(0) species. Apparently, cobalt(I) compounds reduce at 20 °C to metallic cobalt. The black cobalt(0) residue was observed in the NMR tube upon storing the L2iPrCoIICl2/AlMe3 sample during 1 h at 20 °C. Interestingly, complexes 4a and 4b predominate in the sample L2iPrCoIICl2/AlMe3 only in the absence of monomer. The 1H NMR spectrum of the sample L2iPrCoIICl2/AlMe3/ C2H4 ([Al]:[Co]:[C2H4]=10:1:50) recorded just after mixing the reagents at -20 °C displays resonances of the new complex 5 with chemical shifts close to those for the ion pair [L2iPrCoI(η-C2H4)]þ[MeB(C6F5)3]- (3C2H4). The resonances of 4a (4b) were also observed (Figure 3b, Table 1). On the basis of the similarity of the 1H NMR spectra of 5 and 3C2H4, complex 5 can be assigned to the ion pair [L2iPrCoI(ηC2H4)]þ[AlMe3Cl]- (Scheme 2). According to EPR spintrap studies, the Lewis acidity of AlEt2Cl is higher than that of MAO.46 Therefore, the abstraction of methide anion by in situ formed AlMe2Cl and formation of the ion pair 5 seem to be plausible. The decrease of ethylene concentration in the course of polymerization leads to the decrease in concentration of 5 and the growth in concentration of 4a (4b) (Figure 3c). Activation of L3MeCoIICl2 with MAO, AlMe3/ [CPh3][B(C6F5)4], and AlMe3. The 1H NMR spectrum of the catalyst system L3MeCoIICl2/MAO ([Al]:[Co] = 100:1, [Co]=610-3 M, toluene-d8) recorded 5 min after mixing the reagents at -20 °C displays resonances of two types of cobalt(II) complexes (complexes 6 and 7, Figure 5a and Table 2). Warming the sample to 20 °C leads to the disappearance of complex 6, and only complex 7 is observed (Figure 5b). The 1 H NMR spectrum of 7 resembles that of 2 (Tables 1 and 2). (46) Talsi, E. P.; Semikolenova, N. V.; Panchenko, V. N.; Sobolev, A. P.; Babushkin, D. E.; Shubin, A. A.; Zakharov, V. A. J. Mol. Catal. A 1999, 139, 131–137.


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Figure 5. 1H NMR spectra of the L3MeLCoIICl2/MAO ([Al]:[Co] = 100:1, [Co] = 6 10-3 M, toluene-d8) at -20 °C (a) and at 20 °C (b). Asterisks mark still unidentified species, probably outer-sphere ion pairs, containing a Co-Cl instead of a Co-Me moiety.

Therefore, 7 is the outer-sphere ion pair [L3MeCoII(Me)(S)]þ[Me-MAO]-. Complex 6 displays a resonance at δ = 52.5 (6H), characteristic of the AlMe2 moiety in the heterobinuclear ion pairs [LFeII(μ-Me)2AlMe2]þ[Me-MAO]-10 (Figure 5a and Table 2). Hence, 6 is the heterobinuclear ion pair [L3MeCoII(μ-Me)2AlMe2]þ[Me-MAO]- (Scheme 3). Thus, the nature of the substituents in the aryl rings of the bis(imino)pyridine ligand can affect the tendency of the [LCoIIMe]þ cation to bind the particular donor atom. For the L2iPrCoIICl2/MAO system, the structure [L2iPrCoIIMe(S)]þ[Me-MAO]- (S = solvent or vacancy) is preferable at both low and high temperatures, whereas for the L3MeCoIICl2/MAO analogue, the structure [L3MeCoIIMe(S)]þ[Me-MAO]- is preferable at high temperatures, and the structure [L3MeCoII(μ-Me)2AlMe2]þ[Me-MAO]- at low temperatures. In the system L3MeCoIICl2/AlMe3/[CPh3][B(C6F5)4] ([Al]:[Co]:[B]=10:1:1, [Co]=10-2 M, toluene-d8), the heterobinuclear ion pair [L3MeCoII(μ-Me)2AlMe2]þ[B(C6F5)4](60 ) is the major species in the reaction solution at both relatively low (-20 °C) and high (20 °C) temperatures (Figure 6, Table 2). Probably [B(C6F5)4]- is a less bulky counterion than [Me-MAO]- and favors coordination of AlMe3 to the [L3MeCoIIMe]þ cation. The minor signals in Figures 6(a, b) belong to the ion pair [L3MeCoIIMe(S)]þ[B(C6F5)4]- (70 ). The ion pairs 6 (60 ) and 7 (70 ), similar to the ion pairs 2 and 0 2 , reduce at 20 °C to the ion pairs of cobalt(I) (8 and 80 , Table 2). The following structures for these ion pairs can be suggested: [L3MeCoI(S)]þ[Me-MAO]- (8) and [L3MeCoI(S)]þ[B(C6F5)4]- (80 ). Ion pairs 8 and 80 are stable at room temperature. The interaction of L3MeCoIICl2 with AlMe3 at -10 °C leads to reduction of the cobalt(II) and the formation of two complexes of cobalt(I). On the basis of their 1H NMR spectra, they can be assigned to the neutral complexes L3MeCoI(μ-Me)(μ-Cl)AlMe2 (9a) and L3MeCoI(μ-Me)2AlMe2 (9b). As in the case of the L2iPrCoIICl2/AlMe3 system,

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Table 2. 1H NMR Data for the Complexes Observed upon Activation of L3MeCoIICl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3 (toluene-d8)

complex 3Me

II

L Co Cl2a L3MeCoIICl2a 3Me II

[L Co (μ-Me)2AlMe2]þ[Me-MAO][L3MeCoII(μ-Me)2AlMe2]þ[B(C6F5)4][L3MeCoIIMe(S)]þ[Me-MAO]-

6 60 7

[L3MeCoIIMe(S)]þ[B(C6F5)4][L3MeCoI(S)]þ[Me-MAO][L3MeCoI(S)]þ[B(C6F5)4]L3MeCoI(μ-Me)(μ-Cl)AlMe2 L3MeCoI(μ-Me)2AlMe2 [L3MeCoI(η-C2H4)]þ[AlMe3Cl]L3MeCoIMe c L3MeCoICl c

70 8 80 9a 9b 10

A

B

C

D

E

F

T, °C

Py-Hm

Py-Hp

Ar-Hm

Ar-Meo

Ar-Mep

NdC(Me)

þ20 -20 -20 -20 -20 þ20 -20 þ20 þ20 þ10 þ10 þ10 þ20 þ20

110.7 141.5 147.6 147.8 21.7 20.2 21.6 7.61 7.42 7.5 7.6 7.2 7.88 6.90

36.3 49.3 37.5 36.3 52.0 46.4 50.4 8.19 7.89 12.2 12.6 8.35 10.08 9.49

6.5 6.5 34.4 35.3 NA NA NA 6.71 6.66 6.78 6.82 6.54 6.99 6.93

-26.5 -38.1 -7.9 -8.1 -26.3 -22.5 -25.5 1.88 1.80 1.52 1.41 1.57 1.93 2.19

17.0 21.5 43.4 43.4 NA NA NA 1.86 1.76 2.43 2.43 2.03 2.28 2.18

-0.5 -0.5 -5.6 -3.9 -31.7 -24.9 -30.6 -0.71 -0.72 -1.64 -1.85 0.36 -1.21 -0.06

Co-Me

AlMe2

N/O N/O N/O b N/O b N/O

55.2 57.2

N/O N/O

N/O N/O

0.46

a Solution in CH2Cl2. b A broad singlet (δ ≈ 171, Δν1/2 ≈ 600 Hz) was observed at 0 °C after prolonged 1H NMR spectrum accumulation. c The data are from ref 21. N/O, not observed. NA, not reliably assigned.

Scheme 3. Interaction of L3MeCoIICl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3

in the presence of ethylene, the formation of the new complex 10, with proposed structure [L3MeCoI(η-C2H4)]þ[AlMe3Cl]-, is observed (Table 2, Scheme 3). Activation of LtBuCoIICl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3. Complexes observed in the systems LtBuCoIICl2/MAO and LtBuCoIICl2/AlMe3/[CPh3][B(C6F5)4] are similar to those observed in the systems L3MeCoIICl2/MAO and L3MeCoIICl2/AlMe3/[CPh3][B(C6F5)4]. Their structures are presented in Scheme 4, and the parameters of their 1H NMR spectra are collected in Table 3. Activation of LtBuCoIICl2 with AlMe3 affords diamagnetic complexes of cobalt(I), the 1H NMR resonances of which are broadened and less well resolved than in the case of L2iPrCoIICl2 and L3MeCoIICl2 precatalysts. This precludes their assignment; however, addition of ethylene to the sample LtBuCoIICl2/AlMe3 results in the formation of complex 14 with well-resolved signals, which can be

attributed to the ion pair [LtBuCoI(η-C2H4)]þ[AlMe3Cl](Table 3). Activation of LCF3CoCl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3. The 1H NMR spectra of the system LCF3CoIICl2/ MAO show that the ion pair [LCF3CoII(μ-Me)2AlMe2]þ[MeMAO]- (15) is formed at the initial stage of activation at low temperatures (Figure 7a, Table 4). In contrast to the MAObased systems considered above, the heterobinuclear ion pair 15 does not convert to an ion pair of the type [LCoIIMe(S)]þ[Me-MAO]- at room temperature. The halflife of 15 amounts to 1 h at 20 °C. Thus, in the case of L=LCF3, coordination of AlMe3 to the cation [LCoIIMe]þ is more favorable than in the case of L=L2iPr, L3Me, and LtBu. Such precatalysts activated with MAO display only the resonances of the ion pairs [LCoIIMe(S)]þ[Me-MAO]- at 20 °C. Cobalt(II) species formed in the LCF3CoIICl2/MAO system reduce with time into their cobalt(I) counterparts.

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However, the assignment of their 1H NMR resonances is complicated by line broadening caused by rapid reduction of the monovalent cobalt to ferromagnetic cobalt(0). Thus, cobalt(I) species formed in the LCF3CoIICl2/MAO system are less stable to degradation than those formed in the case of the L=L2iPr, L3Me, and LtBu analogues.

Figure 7. 1H NMR spectra (-40 °C) of the samples LCF3CoIICl2/ MAO ([Al]:[Co] = 100:1, [Co] = 5 10-3 M, toluene-d8) (a) and LCF3CoIICl2/AlMe3/[CPh3][B(C6F5)4] ([Al]:[Co]:[B]=10:1:2, [Co] = 10-2 M, toluene-d8) (b) 30 min after storing at -40 °C.

Figure 6. 1H NMR spectra of the L3MeCoIICl2/AlMe3/[CPh3][B(C6F5)4] system ([Al]:[Co]:[B]=10:1:1, [Co]=10-2 M, toluened8) at -20 °C (a) and at 20 °C (b). Asterisks mark still unidentified signals.

Scheme 4. Structures of the Cobalt Species Formed upon Activation of LtBuCoIICl2 with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3/ C2H4

Table 3. 1H NMR Data for the Complexes Observed upon LtBuCoIICl2 Activation with MAO, AlMe3, and AlMe3/[CPh3][B(C6F5)4] (toluene-d8) A complex [LtBuCoII(μ-Me)2AlMe2]þ[Me-MAO][LtBuCoIIMe(S)]þ[Me-MAO]-

11 12

[LtBuCoII(μ-Me)2AlMe2]þ[B(C6F5)4]-

110

[LtBuCoIIMe(S)]þ[B(C6F5)4][LtBuCoI(S)]þ[Me-MAO][LtBuCoI(η-C2H4)]þ[AlMe3Cl]-a

120 13 14

a

B

C

D

E

F

G

H

T, °C

Py-Hm

Py-Hp

Ar-H

C(CH3)3

Ar-H

NdC(Me)

Ar-H

Ar-H

Co-Me

AlMe2

-20 -20 þ20 -20 þ20 þ20 þ20 -30

138.6 ∼17.6 16.5 140.2 118.3 16.5 7.64 7.21

44.1 ∼51.4 44.6 44.6 37.7 44.3 8.27 8.03

30.7 NA NA 31.0 26.9 NA NA NA

-35.8 -14.7 -12.7 -36.4 -28.7 -13.2 1.17/1.14 0.79

-6.0 NA NA -6.3 -4.8 NA NA NA

-10.1 -32.8 -26.5 -10.6 -8.2 -26.2 -0.70 0.85

29.8 NA NA 29.7 25.6 NA 7.23 NA

NA NA NA NA NA NA 6.63 NA

N/O N/O ∼139 N/O N/O N/O

48.3

The signal of η-C2H4 at δ 4.82 was observed. N/O, not observed. NA, not reliably assigned.

48.9 40.9

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Table 4. 1H NMR Data for the Complexes Observed upon Activation of LCF3CoIICl2 with MAO and AlMe3/[CPh3][B(C6F5)4] (toluene-d8) A

B

C

D

E

F

G

complex

T, °C

Py-Hm

Py-Hp

NdC(Me)

Ar-Hm

Ar-Hm

Ar-H

Ar-H

AlMe2

[LCF3CoII(μ-Me)2AlMe2]þ[Me-MAO]-

-40 þ20 -40 þ20 -40

158.3 113.8 156.7 114.9 150.3

67.9 47.1 45.8 37.4 43.6

13.6 9.3 -16.1 -8.4 -15.6

17.8 14.4 37.0 24.7 35.8

-4.5; -4.0 31.0 23.8 29.5

-16.4; -10.3 -6.3 -5.9 -6.2

NA NA NA 10.3 NA

58.5 38.8 56.7

LCF3CoIICl2a

[L

CF3

þ

II

Co (μ-Me)2AlMe2] [B(C6F5)4]

a

-

15 150

Solution in CDCl3. N/O, not observed. NA, not reliably assigned.

Table 5. Ethylene Polymerization Data for LCoIICl2 Complexes with Different Activatorsa content per PE molecule d run

complex

1 2 3 4 5 6 7 8 9 10 11 12 g 13 g

2iPr

L

CoCl2

L3MeCoCl2

LtBuCoCl2 LCF3CoCl2 L2MeCoCl2

activator

yield b

initial activity c

Mw 10-3

Mw/Mn

MAO AlMe3 AlMe3/”B” e MAO MAO (20) AlMe3 AlMe3/”B” MAO AlMe3 MAO AlMe3 MAO AlMe3

1250 750 980 5880 2400 1920 1210 4500 2100 2700 f 1600 f 3100 3600

170 140 110 550 300 235 175 480 370 620 430 80 60

18 18 18 1.7 1.7 1.4 1.2 330 420

2.0 2.0 2.0 1.8 1.9 2.6 1.7 2.5 3.9

0.8 0.8 0.9 1.0 1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.5 1.6

1.8 1.8

1.0 1.0

1.0 1.0

-CH3

-CHdCH2

a Polymerization in toluene at 40 °C, ethylene pressure 5 bar (runs 4 and 8, 2 bar), for 30 min, [Co] = 1.4 10-5 M, [Al]:[Co] = 500:1. b kg PE 3 mol(Co)-1 3 bar-1. c Initial activity, calculated from the PE yield for 2 min of polymerization, in kg PE 3 mol(Co)-1 3 bar-1 3 min-1. d IR spectroscopy data (for runs 4-7 13C NMR data). e “B” = [CPh3][B(C6F5)4] (molar ratio [Co]:[Al]:[B] = 1:100:1). f Liquid oligomers were obtained. g Data of ref 47.

The 1H NMR spectrum of the catalyst system L CoIICl2/AlMe3/[CPh3][B(C6F5)4] recorded 30 min after reaction onset at -40 °C is similar to that observed for the system LCF3CoIICl2/MAO at this temperature and belongs to the ion pair [LCF3CoII(μ-Me)2AlMe2]þ[B(C6F5)4]- (150 ) (Figure 7b, Table 4). This ion pair decays with the half-life of 40 min at 0 °C. The decay is accompanied by the broadening of NMR peaks due to the appearance of cobalt(0) species. Hence, the cobalt species formed in the system LCF3CoIICl2/ AlMe3/[CPh3][B(C6F5)4] are more prone to reduction than their analogues formed in other LCoIICl2/AlMe3/ [CPh3][B(C6F5)4] systems studied. The attempts to identify cobalt(I) species formed in the systems based on L = LCF3 were unsuccessful due to the line broadening. Any attempts to study these systems by 19F NMR were unsuccessful. Correlation of Polymerization and NMR Spectroscopic Data. Data on the catalyst activity for ethylene polymerization by LCoIICl2 (L = L2iPr, LtBu, L3Me) complexes with different activators and the characteristics of the obtained PE samples are collected in Table 5 and Figures 8-11. Like most homogeneous catalysts, the studied systems demonstrate high initial activity that decreases with the increase in polymerization time (Figures 8 and 9). It is possible to distinguish two sections on the kinetic curves: a very sharp drop of the polymerization rate in the first minute of polymerization time and a much more gradual decrease with further duration of the reaction that leads to high yield of the PE in prolonged (30 min) polymerization runs. The value of the initial activity (Table 5) and the deactivation rate depend on the cobalt complex composition (Figure 8). The activator nature weakly affects the shape of the kinetic curve for polymerization over the studied catalysts (Figure 9). The catalyst systems based on LCoIICl2 (L = L2iPr, LtBu, 3Me L ) produce highly linear PE containing about one -CH3 CF3

Figure 8. Rate of the ethylene polymerization vs time for L2iPrCoIICl2 (a), L3MeCoIICl2 (b), and LtBuCoIICl2 (c), activated with MAO (runs 1, 4, and 8 in Table 5).

and one -CHdCH2 group per one PE molecule. The molecular mass of the PE samples obtained with LCoIICl2 depends on the composition of bis(imino)pyridine ligand: with the increase of the steric bulk of a substituent at the ortho-aryl position the MM of the produced PE also increases (Table 5). The obtained PE samples are characterized with rather narrow molecular mass distribution (Mw/Mn was close to 2), depending on the composition of the cobalt complex, the activator nature, and the polymerization conditions. Similar results were obtained for earlier14,47 studied catalyst systems, based on LCoIICl2 (L=L2Me). The molecular structure of the isolated polymers (Table 5, runs 12 and 13) was close to those obtained with the catalysts based on the L3MeCoIICl2 complex (Table 5, runs 4 and 6).

Article

Figure 9. Rate of the ethylene polymerization vs time for L2iPrCoIICl2, activated with MAO (a), AlMe3 (b), and AlMe3/ “B” (c) (runs 1, 2, and 3 in Table 5).

Figure 10. GPC curves for PEs prepared with L3Me CoIICl2, activated with MAO (a) and AlMe3 (b) (runs 4 and 6 in Table 5).

Figure 11. GPC curves for PEs prepared with LtBuCoIICl2, activated with MAO (a) and AlMe3 (b) (runs 8 and 9 in Table 5).

The polymerization rate for the catalyst systems L2MeCoIICl2/MAO and L2MeCoIICl2/AlMe3 was unstable. The Cp (active centers concentration) and kp (propagation rate constant) values for polymerization over the system


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L2MeCoIICl2/MAO were determined.48 It was found that the Cp value decreases by a factor of 1.6 (from 0.23 to 0.14 mol/mol(Co)) with the increase of the polymerization time from 5 to 15 min, whereas the values of the propagation rate constant (kp) were almost independent on the reaction time. Analysis of the obtained data on the Cp and kp values and the molecular mass characteristics of the resulted polymers drew the authors of ref 48 to conclude that the catalyst system L2MeCoIICl2/MAO contains only one type of active center and the reason for its deactivation with the increase of the polymerization time is the reduction of the number of active centers. Similar data on the catalyst behavior of L2MeCoIICl2 and L3MeCoIICl2 activated with MAO makes it possible to assume that similar active centers are formed in these catalyst systems. Comparison of NMR and polymerization data obtained for the catalyst systems based on LCoIICl2 (L=L2iPr, LtBu, L3Me) can provide information about the active centers of these systems. According to the assumption of Gibson et al.,21 the only active species (or their immediate precursors) of the catalyst system L2iPrCoIICl2/MAO is the cobalt(I) ion pair [L2iPrCoI(η-C2H4)]þ[Me-MAO]-. Our NMR studies show that at high [Al]/[Co] ratios (g100) two types of cobalt species are present in the system LiPrCoIICl2/MAO: an ion pair of cobalt(II) (2) and an ion pair of cobalt(I) (3) (Scheme 2). However, in the presence of monomer, complex 2 rapidly converts into 3 ([L2iPrCoI(S)]þ[Me-MAO]-), and, probably, only cobalt species of the type 3C2H4 ([L2iPrCoI(ηC2H4)]þ[Me-MAO]-) exist in the system L2iPrCoIICl2/ MAO/C2H4 in conditions approaching real polymerization. This result is in agreement with the MWD data for the PE produced by the catalyst system L2iPrCoIICl2/MAO, which is typical for single-site catalysts (Mw/Mn = 2.0) (Table 5, run 1). The polymerization activity of L2iPrCoIICl2/AlMe3 and the resulting PE MWD value are very similar to that found for the L2iPrCoIICl2/MAO system (Table 5, run 2). As it was shown above, in the presence of ethylene, complexes of cobalt(I) of the type [L2iPrCoI(η-C2H4)]þ[AlMe3Cl]- (5) were detected in the catalyst system L2iPrCoIICl2/AlMe3. These complexes can be regarded as the immediate precursors of the polymerization active species formed in this system. Thus, the active species (or their precursors) of the L2iPrCoIICl2/MAO and L2iPrCoCl2/AlMe3 systems are similar (ion pairs of cobalt(I), 3C2H4 and 5, respectively), differing only in the nature of counteranion. Polymerization activity of the catalyst system L2iPrCoIICl2/AlMe3/[CPh3][B(C6F5)4] was close to those of the L2iPrCoIICl2/MAO, and the obtained polymers were characterized by their identical GPC curves and MWD values (Table 5, runs 1 and 3). These results evidence that the nature of the counteranion has little effect on the catalyst properties of the cobalt complex. Hence, the initially different complexes of cobalt formed in the L2iPrCoIICl2/MAO and L2iPrCoIICl2/AlMe3 systems convert upon interaction with ethylene into the similar active site precursors 3 (3C2H4) and 5. This explains the close activities of the L2iPrCoIICl2/MAO and L2iPrCoIICl2/AlMe3 (47) Semikolenova, N. V.; Zakharov, V. A.; Echevskaja, L. G.; Matsko, M. A.; Bryliakov, K. P.; Talsi, E. P. Catal. Today 2009, 144, 334–340. (48) Barabanov, A. A.; Semikolenova, N. V.; Bukatov, G. D.; Matsko, M. A.; Zakharov, V. A. J. Polym. Sci., Part B: Polym. Phys. 2008, 50, 326–329.

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systems and formation of the polymers with the same Mw and MWD values. Polymerization activity of the catalyst systems based on L3MeCoIICl2 was noticeably higher than that of the systems obtained with complex L2iPrCoIICl2 (Table 5, runs 4-7). The PE samples obtained with L3MeCoIICl2 activated with MAO, MAO with a reduced content of AlMe3 (MAO (20)), and AlMe3/[CPh3][B(C6F5)4] are characterized by low MW. Narrow MWD (Mw/Mn close to 2) of the studied polymers is characteristic for single-site catalysts. The MWD of the PE produced with the L3MeCoIICl2/AlMe3 catalyst system was slightly broadened (Mw/Mn=2.6, Table 5, run 6). Figure 10 shows the presence of a low molecular mass PE fraction, whereas the position of the peak, corresponding to the main polymer fraction, is close to that of the PE produced with MAO and AlMe3/[CPh3][B(C6F5)4] activators. Formation of the low molecular mass PE is not clear, and, possibly, it can be attributed to some fluctuations in the course of the polymerization run or more facile transmetalation in the case of TMA than MAO. Nevertheless, it is possible to conclude that upon activation of L3MeCoIICl2 with MAO and AlMe3, similar cobalt(I) species [L3MeCoI(η-C2H4)]þ[Me-MAO](8C2H4) and [L3MeCoI(η-C2H4)]þ[AlMe3Cl]- (10) dominate in the reaction solution, determining formation of PE with close molecular structure. LtBuCoIICl2/MAO and LtBuCoIICl2/AlMe3 systems produce PE with noticeably higher MW (Mw=(3.3-4.2) 105, Table 5, runs 8 and 9) due to the presence of a bulky t-Bu substituent in the bis(imino)pyridyl ligand, hindering the chain transfer reactions. Figure 11 shows that the positions of a peak corresponding to the main polymer fraction of PEs produced with LtBuCoIICl2/MAO and LtBuCoIICl2/AlMe3 are close. The moderate increase in the polydispersity value can be attributed to the formation of a small amount of high molecular mass PE. Both catalysts have close values of the initial activities and produce PE in similar yields. These data suggest that the close catalyst properties of the LtBuCoIICl2/ MAO and LtBuCoIICl2/AlMe3 systems are determined by close composition of the direct precursors of the active centers (cobalt(I) species denoted as 13 (13C2H4) and 14 (Scheme 4)), as was shown above for the catalyst systems LCoIICl2/MAO and LCoIICl2/AlMe3 (L=L2iPr and L3Me). In contrast to the catalysts described, the systems formed by activation of LCF3CoIICl2 with MAO and AlMe3 proved to be highly active catalyst for ethylene oligomerization (Table 5, runs 10 and 11). These catalysts demonstrated high initial activity that drops with time more sharply than in the case of the catalyst systems based on LCoIICl2 (L = L2iPr, LtBu, L3Me) and after 10 min of polymerization were almost deactivated. This corresponds to the NMR data, showing that the intermediates formed in the LCF3CoIICl2/MAO (AlMe3) system are less stable to degradation than those formed in the case of the LCoIICl2/MAO (L = L2iPr, LtBu, L3Me) analogues.

Conclusions The cobalt(II) and cobalt(I) species, formed upon activation of LCoIICl2 (L=L3Me, L2iPr, LtBu, and LCF3) with MAO, AlMe3/[CPh3][B(C6F5)4], and AlMe3 were monitored by 1H, 2 H, and 19F NMR spectroscopy. It was shown that the ion pairs [LCoII(μ-Me)2AlMe2]þ[A]-, [LCoIIMe(S)]þ[A]-, and [LCoI(S)]þ[A]- can be observed in the catalyst systems LCoIICl2/MAO and LCoIICl2/AlMe3/[CPh3][B(C6F5)4]

Soshnikov et al.

([A]-=[Me-MAO]- or [B(C6F5)4]-; S=toluene or vacancy), whereas neutral complexes LCoI(μ-Me)(μ-Cl)AlMe2 and LCoI(μ-Me)2AlMe2 predominate in the catalyst systems LCoIICl2/AlMe3. Addition of monomer (C2H4) plays the key role in formation of the direct precursors of the polymerization active centers of the catalyst systems based on LCoIICl2. In the case of the LCoIICl2/MAO systems, addition of ethylene results in rapid reduction of cobalt(II) to cobalt(I), and only the ion pairs of the type [LCoI(S)]þ[A]- are present in the reaction solution at 20 °C. In the case of the LCoIICl2/ AlMe3 systems, in the presence of ethylene an ion pair with proposed structure [LCoI(η-C2H4)]þ[AlMe3Cl]- is the major cobalt species in the reaction solution. Similar intermediates (ion pairs of cobalt(I)) are present in the systems LCoIICl2/ MAO/C2H4 and LCoIICl2/AlMe3/C2H4. The results obtained can explain the close polymerization results (similar activity and PE structure) obtained with the catalyst systems based on LCoIICl2 complexes activated with MAO and AlMe3.

Experimental Section General Experimental Details. Toluene was dried over molecular sieves (4 A˚), purified by refluxing over sodium metal, and distilled under dry argon. Toluene-d8 was dried over molecular sieves (4 A˚), degassed in vacuo, and stored under dry argon. All experiments were carried out in sealed high-vacuum systems using break-seal techniques. LCoIICl2 (L=L3Me, L2iPr, LtBu, and LCF3)6,20 were synthesized as described. Trimethylaluminum (AlMe3) and [CPh3][B(C6F5)4] were purchased from Aldrich. Methylaluminoxane (MAO) was obtained from Crompton GmbH (Bergkamen) as a toluene solution (total Al concentration 1.8 M, Al as AlMe3 0.5 M). For the purposes of this paper, MAO (or MAO(20)) with total Al content of 40 wt % and 10 mol % of Al as AlMe3 was obtained as a solid product by solvent removal in vacuo at 20 °C from commercial MAO. 1H and 19F NMR spectra were recorded at 250.130 and 62.89 MHz, on a Bruker DPX-250 MHz NMR spectrometer. For the 2H NMR spectra, a Bruker Avance-400 MHz NMR spectrometer was used. EPR spectra were measured on a Bruker ER-200D spectrometer at 9.3 GHz at room temperature. NMR and EPR Sample Preparation. Sample preparation was carried out in sealed high-vacuum systems using break-seal techniques. Calculated quantities of the cobalt complexes were weighed, evacuated, and transferred into the NMR tube (length = 200 mm, d =5 mm). Toluene-d8, AlMe3, or MAO (20) was added after LCoIICl2 cooling in liquid nitrogen (77 K). In the case of [CPh3][B(C6F5)4], this compound was transferred to the NMR tube (as a solution in the CH2Cl2 with subsequent solvent removal) before LCoIICl2 addition. Ethylene Polymerization Procedure. Polymerization was performed in a steel 1 L autoclave. Precatalysts (2.0 10-6 mol) were introduced into the autoclave in a vacuum-sealed glass ampule. The reactor was evacuated at 80 °C, cooled to 20 °C, and charged with the solution of a calculated cocatalyst amount in 150 cm3 of toluene. After setting up the polymerization temperature (40 °C) and ethylene pressure (5 bar), the reaction was started by breaking the ampule of the complex. During the polymerization time (30 min), ethylene pressure, stirring speed, and temperature were maintained constant. The experimental unit was equipped with an automatic computer-controlled system for ethylene feed and recording of the ethylene consumption. Polymer MW and MWD Measurements. Weight-average (Mw) and number-average (Mn) molecular weights and molecular weight distributions (Mw/Mn) were obtained by the GPC method with a Waters-150 at 150 °C with trichlorobenzene as a solvent.

Acknowledgment. This work was supported by the Russian Fund of Basic Research, Grant 09-03-00485.

Article

The authors are grateful to A. V. Golovin for help with NMR spectroscopic studies. We are also grateful to L. G. Echevskaja and M. A. Matsko for help in the analysis of polymer MWD.


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Supporting Information Available: NMR details for the complexes observed (JH-H coupling constants, line halfwidths). This material is available free of charge via the Internet at http://pubs.acs.org.

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